MARC is an acronym for MAchine-Readable Cataloging.  The five MARC 21 communication formats, Bibliographic Data, Authority Data, Holdings Data, Classification Data, and Community Information, are widely used standards for the representation and exchange of bibliographic, authority, holdings, classification, and community information data in machine-readable form.

Each MARC format provides detailed field descriptions and guidelines for applying the defined content designation and identifies conventions to be used to insure input consistency.

The content of the data elements that comprise a MARC record is usually defined by standards outside the formats.  Examples are the Library of Congress Classification (LCC), the Dewey Decimal Classification (DDC), or other classification schemes used by the organization that creates the record.  The content of certain coded data elements is defined in the MARC formats (e.g., the Leader, field 00, subfield $w in the 4XX and 5XX tracing fields).

A section in the documentation entitled scheme-specific conventions describes coding practices for the two major classification schemes:  the Library of Congress Classification (LCC) and the Dewey Decimal Classification (DDC).

Classification data may be used for online public catalog retrieval systems, online systems for library classifiers (e.g., for machine-assisted classification), systems for the maintenance and development of classification schedules, validation of classification numbers assigned to bibliographic records, and linking to MARC authority records.  Wherever possible, classification data elements were designed to be generic, i.e., usable for any classification scheme.  Data element features were designed to accommodate the two major classification schemes in use in the United States:  Dewey Decimal Classification (DDC) and Library of Congress Classification (LCC).

See also:

Classification Data Format:  Introduction